1. Field of the Invention
This invention relates to a process for the preparation of copolyesters. More particularly, it relates to the preparation of copolyesters having random sequence distributions from the interaction of two or more polyesters or one or more polyesters and polyester precursors in a melt phase in the presence of triarylphosphine catalysts.
2. Description of the Prior Art
Phosphites and phosphates are known to be useful as polyester stabilizers as disclosed, for example, in British Pat. No. 1,060,401 (published Mar. 1, 1967). However, these compounds are generally poor catalysts for melt phase copolyester formation as shown below in Examples 22 and 28.
U.S. Pat. No. 3,053,809 (issued Sept. 11, 1962 to Linville) discloses the use of metal-phosphine complexes as catalysts for linear polyester formation. This patent specifically teaches (see Column 1, lines 48-52) that the organic phosphines alone do not act as catalysts to any substantial extent.
It is known that copolyesters can be prepared by a melt phase process as disclosed, for example, in U.S. Pat. No. 3,772,405 (issued Nov. 13, 1973 to Hamb). This patent describes a preparation of copolyesters by the interaction of a polyester, a dicarboxylic acid and an aromatic diester with or without standard transesterification catalysts, e.g., alkali metals, alkaline earth metals, titanium, tin, antimony and lead.
However, in many instances, copolyesters prepared by the Hamb process have block sequence distributions. This means that the units making up the copolyesters tend to cluster in blocks of particular units rather than being randomly distributed throughout the copolyester chain. Generally, block copolyesters are not easily formulated into useful products because they tend to have higher melting points than is desirable. This tendency in polyesters to cluster in blocks also causes the reaction mixture of polymers to become increasingly heterogeneous as the reaction proceeds. In other words, the block polymers formed become less soluble in each other. This lack of solubility causes clumps of polymer to form until eventually, the entire mass solidifies. This problem is especially prominent for copolyesters prepared from polyesters formed from alkylene glycols higher in molecular weight than ethylene glycol.
To overcome this problem of heterogeneity, workers in the art have tried to prepare copolyesters by adding the reactants gradually to the reaction vessel. However, this gradual addition lengthens reaction times. Extended reaction times can be disadvantageous for copolyesters susceptible to decomposition from prolonged exposure to high reaction temperatures.
Hence, there is a need in the art for a melt phase process for preparing copolyesters having random sequence distributions which is not limited, in practicality, to a limited group of starting materials. Further, it is desirable that these copolyesters be prepared over shorter reaction times in order to avoid their prolonged exposure to high reaction temperatures.